1. Field of the Invention
The present invention relates to a wireless communication apparatus, system, method, and program using multi-user MIMO transmission that adopts preceding.
2. Description of the Related Art
A MIMO (Multiple Input Multiple Output) transmission method is known as a technique of increasing the channel capacity of wireless communications by causing each of transmitters and receivers to perform transmission/receiving using a plurality of antennas and a plurality of radio units.
The MIMO transmission can increase the channel capacity in accordance with the number of antennas and radio units of transmitters and receivers. However, for example, a portable compact terminal can have only a limited number of antennas and radio units in view of its size and power consumption. This hinders the channel capacity from unlimitedly increasing, so restrictions on the device arrangement limit the channel capacity.
A transmission method called multi-user MIMO or SDMA (Spatial Division Multiple Access) is known, which communicates with a plurality of wireless apparatuses simultaneously using a single frequency. In the multi-user MIMO, the antennas and radio units of wireless apparatuses communicating simultaneously virtually form MIMO channels using a lot of antennas and radio units. For this reason, even when each wireless apparatus has only a limited number of antennas and radio units, a large channel capacity is available.
Methods of multiplexing a plurality of wireless apparatuses using multi-user MIMO in downlink channels from a base station or an access point to wireless communication terminals can roughly be classified into two types: methods using only beamforming and methods using both beamforming and preceding. Examples of multiplexing using only beamforming are methods of dividing space using a plurality of sector antennas having different directivities and causing them to communicate with different wireless communication terminals, and methods of adaptively forming beams for respective wireless communication terminals using an adaptive array and multiplexing the plurality of beams. The multiplexing methods using only beamforming are almost the same as transmission techniques using single-user MIMO transmission except that the signals of different wireless communication terminals are transmitted at the same frequency in the same time. Additionally, receiving can be done using the same scheme as in single-user MIMO transmission. This easily implements multi-user MIMO transmission. However, the multiplexing using only beamforming is significantly affected by correlation of propagation paths between the wireless communication terminals. If the correlation is high because of, e.g., wireless communication terminals in close vicinity, the performance considerably degrade. To the contrary, the methods using preceding together adopt coding according to modulation symbols (e.g., R. F. H. Fischer, C. Windpassinger, A. Lamper, and J. B. Huber, “MIMO preceding for decentralized receivers”, In proc. ISIT 2002, Lausanne, Switzerland, June 2002.; B. M. Hochwald, C. B. Peel, and A. Lee Swindlehurst, “A vector perturbation technique for near-capacity multiantenna multiuser communication—part II: perturbation”, IEEE Trans. Commun. vol. 53, No. 3, pp. 537-544, March 2005). Although the operation load in transmission is heavy, these methods are hardly influenced by correlation of propagation paths between terminals. It is therefore possible to obtain an outstanding channel capacity increasing effect by multiplexing as compared to multiplexing by only beamforming.
However, when Tomlinson-Harashima preceding in R. F. H. Fischer, C. Windpassinger, A. Lamper, and J. B. Huber, “MIMO preceding for decentralized receivers”, In proc. ISIT 2002, Lausanne, Switzerland, June 2002 or vector perturbation in B. M. Hochwald, C. B. Peel, and A. Lee Swindlehurst, “A vector perturbation technique for near-capacity multiantenna multiuser communication—part II: perturbation”, IEEE Trans. Commun. vol. 53, No. 3, pp. 537-544, March 2005 is applied as a preceding scheme, the position of the constellation point of a signal to be received by a receiver may shift due to the influence of a signal added by preceding. For this reason, the digital signal processing unit of the receiver needs to have a wider dynamic range than that in a traditional receiver without preceding, resulting in an increase in the device scale and power consumption.
As described above, a large device scale and high power consumption pose problems in the conventional wireless communication apparatus. There are also challenges to higher system throughput, shorter waiting time, smaller crest factor of transmission signals, and improved coverage that enables communication at a predetermined throughput or more (reduction of dead zones).